PROJECT SUMMARY Elastin, the major component of vascular extracellular matrix, provides arteries the elastic recoil necessary for normal heart function. Both human genetics and animal studies have identified several extracellular matrix proteins as important for proper elastic fiber formation, however the process by which elastin assembles into a functional fiber remains unclear. In addition to elastin, two proteins required in this regard are lysyl oxidase (LOX) and Fibulin-4 (FBLN4), which will be the focus of this proposal. Mice deficient in Fbln4 die shortly after birth due to cardiopulmonary failure from the absence of intact elastic fibers. Recently humans with a multisystem disorder known as autosomal recessive cutis laxa type 1B (ARCL 1B) were identified to carry mutations in FBLN4. Patients with ARCL1B develop inelastic skin, aortic aneurysms, arterial tortuosity, pulmonary emphysema and skeletal abnormalities. Using a mouse model harboring a Fbln4 mutation (E57K) seen in humans, our preliminary studies showed significant elastic fiber fragmentation and wall disarray in large arteries, ascending aortic aneurysms, arterial stiffness and hypertension in Fbln4E57K mice. Surprisingly however, elastic fibers in resistance arteries were unaffected, suggesting the process of elastic fiber formation, which was previously thought to be the same in all tissues, may differ between elastic and muscular arteries. The studies proposed herein will address the role FBLN4 plays in maintaining arterial wall integrity and the mechanisms by which its disruption leads to aneurysmal disease. The specific aims are to: (1) Define the functional interaction between FBLN4 and LOX and determine how mutations in FBLN4 lead to abnormal elastic fiber formation and vessel wall weakness; (2) Identify mechanism(s) underlying differences in elastic fiber assembly and arterial wall integrity between Fbln4E57K elastic and muscular arteries; and (3) Investigate alterations in signaling pathways and blood pressure as potential therapeutic targets for aneurysm formation in Fbln4E57K mice. Addressing these aims will not only enhance our understanding of the role of FBLN4 in vessel wall maturation, but will also identify potential therapeutic targets for aneurysmal disease.